Method of Measuring a Structure, Inspection Apparatus, Lithographic System and Device Manufacturing Method

ABSTRACT

An overlay metrology target (T) is formed by a lithographic process. A first image (740(0)) of the target structure is obtained using with illuminating radiation having a first angular distribution, the first image being formed using radiation diffracted in a first direction (X) and radiation diffracted in a second direction (Y). A second image (740(R)) of the target structure using illuminating radiation having a second angular illumination distribution which the same as the first angular distribution, but rotated 90 degrees. The first image and the second image can be used together so as to discriminate between radiation diffracted in the first direction and radiation diffracted in the second direction by the same part of the target structure. This discrimination allows overlay and other asymmetry-related properties to be measured independently in X and Y, even in the presence of two-dimensional structures within the same part of the target structure.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a Divisional of U.S. patent application Ser. No. 15/839,285,filed Dec. 12, 2017, which claims priority of European Application No.16204457, filed Dec. 15, 2016 which are incorporated herein in theirentireties by reference.

BACKGROUND Field of the Invention

The present invention relates to methods and apparatus for metrologyusable, for example, in the manufacture of devices by lithographictechniques, and to methods of manufacturing devices using lithographictechniques.

Background Art

A lithographic apparatus is a machine that applies a desired patternonto a substrate, usually onto a target portion of the substrate. Alithographic apparatus can be used, for example, in the manufacture ofintegrated circuits (ICs). In that instance, a patterning device, whichis alternatively referred to as a mask or a reticle, may be used togenerate a circuit pattern to be formed on an individual layer of theIC. This pattern can be transferred onto a target portion (e.g.including part of a die, one die, or several dies) on a substrate (e.g.,a silicon wafer). Transfer of the pattern is typically via imaging ontoa layer of radiation-sensitive material (resist) provided on thesubstrate. In general, a single substrate will contain a network ofadjacent target portions (known as fields) that are successivelypatterned.

In lithographic processes, it is desirable frequently to makemeasurements of the structures created, e.g. for process control andverification. Various tools for making such measurements are known,including scanning electron microscopes, which are often used to measurecritical dimension (CD), and specialized tools to measure overlay, theaccuracy of alignment of two layers in a device. Recently, various formsof scatterometers have been developed for use in the lithographic field.These devices direct a beam of radiation onto a target and measure oneor more properties of the scattered radiation—e.g. intensity at a singleangle of reflection as a function of wavelength; intensity at one ormore wavelengths as a function of reflected angle; or polarization as afunction of reflected angle—to obtain a diffraction “spectrum” fromwhich a property of interest of the target can be determined.

Examples of known scatterometers include angle-resolved scatterometersof the type described in US2006033921A1 and US2010201963A1. The targetsused by such scatterometers are relatively large gratings, e.g. 40 μm by40 μm, and the measurement beam generates a spot that is smaller thanthe grating (i.e., the grating is underfilled). In addition tomeasurement of feature shapes by reconstruction, diffraction basedoverlay can be measured using such apparatus, as described in publishedpatent application US2006066855A1. Diffraction-based overlay metrologyusing dark-field imaging of the diffraction orders enables measurementof overlay and other parameters on smaller targets. These targets can besmaller than the illumination spot and may be surrounded by productstructures on a substrate. The intensities from the environment productstructures can efficiently be separated from the intensities from theoverlay target with the dark-field detection in the image-plane.

Examples of dark field imaging metrology can be found in patentapplications US20100328655A1 and US2011069292A1 which documents arehereby incorporated by reference in their entirety. Further developmentsof the technique have been described in published patent publicationsUS20110027704A, US20110043791A, US2011102753A1, US20120044470A,US20120123581A, US20120242970A1, US20130258310A, US20130271740A andWO2013178422A1. Typically in these methods it is desired to measureasymmetry as a property of the target. Targets can be designed so thatmeasurement of asymmetry can be used to obtain measurement of variousperformance parameters such as overlay, focus or dose. Asymmetry of thetarget is measured by detecting differences in intensity betweenopposite portions of the diffraction spectrum using the scatterometer.For example, the intensities of +1 and −1 diffraction orders may becompared, to obtain a measure of asymmetry.

In these known techniques, appropriate illumination modes and imagedetection modes are used to obtain the +1 and −1 diffraction orders fromperiodic structures (gratings) within the target. Comparing theintensity of these opposite diffraction orders provides a measurement ofasymmetry of the structure. Comparing the measured asymmetry for two ormore gratings with known bias values provides a measurement of overlayin the process by which the structures were formed. By appropriatedesign of the targets, process performance parameters other than overlaycan also be measured by the same technique, for example focus and dose.

Grating structures in metrology targets of the type described may besegmented in a direction other than their main direction of periodicity.Reasons for this segmentation may be to induce asymmetry-related effectsto allow measurement of properties other than overlay, in the mannerjust mentioned. Other reasons for this segmentation may be to make thegrating structures more “product-like”, so that they are printed withpatterning performance more like the product structures that areprimarily of interest. Grating structures may simply be completelytwo-dimensional in layout, for example to resemble an array of contactholes or pillars. Nevertheless, overlay, focus or other parameters ofthe performance of the patterning process are normally controlled andmeasured separately in two or more directions, typically the X and Ydirections defined relative to the substrate.

In order to reduce measurement time, known apparatuses for dark-fieldmetrology have apertures and detection systems configured to detectsimultaneously the radiation diffracted from component gratings in bothX and Y directions, and to detect these different directions ofdiffraction independently. Thus, the need for separate detection stepsin X and Y orientation is avoided. Examples of such techniques areincluded in the prior patent publications mentioned above, and also forexample in unpublished patent application EP16157503.0. Unfortunately,where the grating structures in a metrology target are two-dimensionallystructured, either being fully two-dimensional gratings or having somekind of segmentation in the orthogonal to their main direction ofperiodicity, diffraction by a structure in the orthogonal directionbecomes mixed with diffraction in the main direction, and the separatemeasurements become subject to noise or cross-talk. Consequently, aparticular method or apparatus may be unusable with such targets, or atleast the operating mode has to be changed. To exacerbate this problem,in general it may not even be known to the operator of the metrologyapparatus, whether metrology targets under investigation havetwo-dimensional properties of the type described.

SUMMARY OF THE INVENTION

The present invention in a first aspect aims to allow independentmeasurement of asymmetry of targets in two directions using availabletechniques, even when target structures may be two-dimensional innature. The present invention in another aspect aims to allowrecognition of two-dimensional character in metrology targets, withoutrelying on advance information.

The invention in a first aspect provides A method of determining aproperty of at least a first part of a target structure formed by alithographic process, the method being based on radiation diffracted byperiodic features within the target structure and including thefollowing steps:

(a) using a detection system to form a first image of the targetstructure when illuminated with radiation having a first angulardistribution, the first image being formed using a selected portion ofradiation diffracted by the target structure in a first direction and aselected portion of radiation diffracted by the target structure in asecond direction, said first and second directions being definedrelative to the target structure and being non-parallel;

(b) using the detection system to form a second image of the targetstructure when illuminated with radiation having a second angularillumination distribution, the first and second angular illuminationprofiles being oriented differently to one another, relative to thetarget structure;

(c) combining intensity values from the first image and the second imageso as to discriminate between radiation diffracted in the firstdirection by a first part of the target structure and radiationdiffracted in the second direction by the same first part of the targetstructure; and

(d) based at least partly on the discrimination made in step (c),determining the property of said first part of the structure.

In a case where a part of the target has periodic structure intwo-dimensions, the discrimination in step (c) allows the presence ofthis two-dimensional structure to be detected. The discrimination instep (c) can be used instead or in addition to calculate asymmetry ofeach part of the target structure in the first direction, whiledisregarding radiation diffracted in the second direction.

The method may further comprise calculating a performance parameter ofsaid lithographic process based on the asymmetry determined by themethod for a plurality of periodic structures. The performance parametermay be, for example, overlay, focus or dose.

The invention further provides an inspection apparatus for measuring aproperty of a target structure formed by a lithographic process on oneor more substrates, the inspection apparatus comprising:

-   an illumination system operable to illuminate a target structure at    different times with radiation having a first angular distribution    and a second angular distribution;-   a detection system operable to form one or more images of the target    structure using selected portions of radiation diffracted by the    target structure;-   a controller for controlling the illumination system and the    detection system (a) to form a first image of the target structure    when illuminated with radiation having a first angular distribution,    the first image being formed using a selected portion of radiation    diffracted by the target structure in a first direction and a    selected portion of radiation diffracted by the target structure in    a second direction, said first and second directions being defined    relative to the target structure and being non-parallel and (b) to    form a second image of the target structure when illuminated with    radiation having a second angular illumination distribution, the    first and second angular illumination profiles being oriented    differently to one another, relative to the target structure.

The inspection apparatus may further comprise a processor configured (c)to combine intensity values from the first image and the second image soas to discriminate between radiation diffracted in the first directionby a first part of the target structure and radiation diffracted in thesecond direction by the same first part of the target structure and (d)based at least partly on the discrimination made in step (c), to theproperty of said first part of the structure.

The invention in another aspect provides various target structures foruse in a method according to the invention as set forth above. In oneembodiment, said target structure includes at least three parts eachperiodic in both a first direction and a second direction, the first andsecond directions being non-parallel, at least two parts among saidthree parts having different programmed bias values in the firstdirection and at least two parts among said three parts having differentprogrammed bias values in the second direction.

Because the method of the invention discriminates between asymmetry indifferent directions, it allows the number of biased gratings to bereduced, relative to a conventional method.

The invention in another aspect provides a processing device arranged toreceive at least first and second images of a target structure and toderive a measurement of one or more properties of one or more parts of atarget structure by performing the steps (c) and (d) in the methodaccording to the first aspect of the invention as set forth above.

The invention further provides one or more computer program productscomprising machine readable instructions for causing a programmableprocessing device to implement one or more aspects of the invention asset forth above. The machine readable instructions may be embodied, forexample, in a non-transitory storage medium.

The invention further provides a lithographic system including alithographic apparatus and an inspection apparatus according to theinvention, as set forth above.

The invention further provides a method of manufacturing devices whereina device pattern is applied to a series of substrates using alithographic process, the method including measuring one or moreproperties of at one or more structures formed as part of or beside saiddevice pattern on at least one of said substrates using a methodaccording to the invention as set forth above, and controlling thelithographic process for later substrates in accordance with the resultof the measuring.

Further features and advantages of the invention, as well as thestructure and operation of various embodiments of the invention, aredescribed in detail below with reference to the accompanying drawings.It is noted that the invention is not limited to the specificembodiments described herein. Such embodiments are presented herein forillustrative purposes only. Additional embodiments will be apparent topersons skilled in the relevant art(s) based on the teachings containedherein.

BRIEF DESCRIPTION OF THE DRAWINGS/FIGURES

Embodiments of the invention will now be described, by way of example,with reference to the accompanying drawings in which:

FIG. 1 depicts a lithographic apparatus together with other apparatusesforming a production facility for semiconductor devices;

FIG. 2 illustrates schematically (a) an inspection apparatus adapted toperform angle-resolved scatterometry and dark-field imaging inspectionmethods in accordance with some embodiments of the invention and (b) anenlarged detail of the diffraction of incident radiation by a targetgrating in the apparatus of FIG. 2(a);

FIG. 3 illustrates (a) a segmented illumination profile, (b) theproduction of diffraction signals in different directions under thesegmented illumination profile and (c) the layout of a prism device in asegmented detection system, all in the operation of one embodiment ofthe inspection apparatus of FIG. 2;

FIG. 4 illustrates a composite metrology target including a number ofcomponent gratings (a) in a case where each component grating isperiodic in only one direction and (b) in a case where each componentgrating is or may be periodic in two directions;

FIG. 5 illustrates a multiple image of the target of FIG. 4, captured bythe apparatus of FIG. 4 with spatial separate of diffraction orders;

FIG. 6 illustrates (a)-(f) the production of diffraction signals similarto FIG. 3, but using a second illumination profile to discriminatebetween radiation diffracted in different directions by the same part ofthe target, in accordance with the principles of the present disclosure;

FIG. 7 illustrates dark-field images obtained using the first and secondillumination profiles of FIG. 6;

FIG. 8 is a flowchart of a method of measuring a property of a targetstructure and a method of controlling a lithographic process using theprinciples of FIG. 6; and

FIG. 9 illustrates (a)-(b) some modified target layouts designed for usein the method of FIG. 8.

DETAILED DESCRIPTION

Before describing embodiments of the invention in detail, it isinstructive to present an example environment in which embodiments ofthe present invention may be implemented.

FIG. 1 at 100 shows a lithographic apparatus LA as part of an industrialfacility implementing a high-volume, lithographic manufacturing process.In the present example, the manufacturing process is adapted for themanufacture of semiconductor products (integrated circuits) onsubstrates such as semiconductor wafers. The skilled person willappreciate that a wide variety of products can be manufactured byprocessing different types of substrates in variants of this process.The production of semiconductor products is used purely as an examplewhich has great commercial significance today.

Within the lithographic apparatus (or “litho tool” 100 for short), ameasurement station MEA is shown at 102 and an exposure station EXP isshown at 104. A control unit LACU is shown at 106. In this example, eachsubstrate visits the measurement station and the exposure station tohave a pattern applied. In an optical lithographic apparatus, forexample, a projection system is used to transfer a product pattern froma patterning device MA onto the substrate using conditioned radiationand a projection system. This is done by forming an image of the patternin a layer of radiation-sensitive resist material.

The term “projection system” used herein should be broadly interpretedas encompassing any type of projection system, including refractive,reflective, catadioptric, magnetic, electromagnetic and electrostaticoptical systems, or any combination thereof, as appropriate for theexposure radiation being used, or for other factors such as the use ofan immersion liquid or the use of a vacuum. The patterning MA device maybe a mask or reticle, which imparts a pattern to a radiation beamtransmitted or reflected by the patterning device. Well-known modes ofoperation include a stepping mode and a scanning mode. As is well known,the projection system may cooperate with support and positioning systemsfor the substrate and the patterning device in a variety of ways toapply a desired pattern to many target portions across a substrate.Programmable patterning devices may be used instead of reticles having afixed pattern. The radiation for example may include electromagneticradiation in the deep ultraviolet (DUV) or extreme ultraviolet (EUV)wavebands. The present disclosure is also applicable to other types oflithographic process, for example imprint lithography and direct writinglithography, for example by electron beam.

The lithographic apparatus control unit LACU controls the movements andmeasurements of various actuators and sensors, causing the apparatus LAto receive substrates W and reticles MA and to implement the patterningoperations. LACU also includes signal processing and data processingcapacity to implement desired calculations relevant to the operation ofthe apparatus. In practice, control unit LACU will be realized as asystem of many sub-units, each handling the real-time data acquisition,processing and control of a subsystem or component within the apparatus.

Before the pattern is applied to a substrate at the exposure stationEXP, the substrate is processed in at the measurement station MEA sothat various preparatory steps may be carried out. The preparatory stepsmay include mapping the surface height of the substrate using a levelsensor and measuring the position of alignment marks on the substrateusing an alignment sensor. The alignment marks are arranged nominally ina regular grid pattern. However, due to inaccuracies in creating themarks and also due to deformations of the substrate that occurthroughout its processing, the marks deviate from the ideal grid.Consequently, in addition to measuring position and orientation of thesubstrate, the alignment sensor in practice must measure in detail thepositions of many marks across the substrate area, if the apparatus isto print product features at the correct locations with very highaccuracy. The apparatus may be of a so-called dual stage type which hastwo substrate tables, each with a positioning system controlled by thecontrol unit LACU. While one substrate on one substrate table is beingexposed at the exposure station EXP, another substrate can be loadedonto the other substrate table at the measurement station MEA so thatvarious preparatory steps may be carried out. The measurement ofalignment marks is therefore very time-consuming and the provision oftwo substrate tables enables a substantial increase in the throughput ofthe apparatus. If the position sensor IF is not capable of measuring theposition of the substrate table while it is at the measurement stationas well as at the exposure station, a second position sensor may beprovided to enable the positions of the substrate table to be tracked atboth stations. Lithographic apparatus LA for example is of a so-calleddual stage type which has two substrate tables WTa and WTb and twostations—an exposure station and a measurement station—between which thesubstrate tables can be exchanged.

Within the production facility, apparatus 100 forms part of a “lithocell” or “litho cluster” that contains also a coating apparatus 108 forapplying photosensitive resist and other coatings to substrates W forpatterning by the apparatus 100. At an output side of apparatus 100, abaking apparatus 110 and developing apparatus 112 are provided fordeveloping the exposed pattern into a physical resist pattern. Betweenall of these apparatuses, substrate handling systems take care ofsupporting the substrates and transferring them from one piece ofapparatus to the next. These apparatuses, which are often collectivelyreferred to as the “track”, are under the control of a track controlunit which is itself controlled by a supervisory control system SCS,which also controls the lithographic apparatus via lithographicapparatus control unit LACU. Thus, the different apparatuses can beoperated to maximize throughput and processing efficiency. Supervisorycontrol system SCS receives recipe information R which provides in greatdetail a definition of the steps to be performed to create eachpatterned substrate.

Once the pattern has been applied and developed in the litho cell,patterned substrates 120 are transferred to other processing apparatusessuch as are illustrated at 122, 124, 126. A wide range of processingsteps is implemented by various apparatuses in a typical manufacturingfacility. For the sake of example, apparatus 122 in this embodiment isan etching station, and apparatus 124 performs a post-etch annealingstep. Further physical and/or chemical processing steps are applied infurther apparatuses, 126, etc. Numerous types of operation can berequired to make a real device, such as deposition of material,modification of surface material characteristics (oxidation, doping, ionimplantation etc.), chemical-mechanical polishing (CMP), and so forth.The apparatus 126 may, in practice, represent a series of differentprocessing steps performed in one or more apparatuses.

As is well known, the manufacture of semiconductor devices involves manyrepetitions of such processing, to build up device structures withappropriate materials and patterns, layer-by-layer on the substrate.Accordingly, substrates 130 arriving at the litho cluster may be newlyprepared substrates, or they may be substrates that have been processedpreviously in this cluster or in another apparatus entirely. Similarly,depending on the required processing, substrates 132 on leavingapparatus 126 may be returned for a subsequent patterning operation inthe same litho cluster, they may be destined for patterning operationsin a different cluster, or they may be finished products to be sent fordicing and packaging.

Each layer of the product structure requires a different set of processsteps, and the apparatuses 126 used at each layer may be completelydifferent in type. Further, even where the processing steps to beapplied by the apparatus 126 are nominally the same, in a largefacility, there may be several supposedly identical machines working inparallel to perform the step 126 on different substrates. Smalldifferences in set-up, or faults between these machines can mean thatthey influence different substrates in different ways. Even steps thatare relatively common to each layer, such as etching (apparatus 122) maybe implemented by several etching apparatuses that are nominallyidentical but working in parallel to maximize throughput. In practice,moreover, different layers require different etch processes, for examplechemical etches, plasma etches, according to the details of the materialto be etched, and special requirements such as, for example, anisotropicetching.

The previous and/or subsequent processes may be performed in otherlithography apparatuses, as just mentioned, and may even be performed indifferent types of lithography apparatus. For example, some layers inthe device manufacturing process which are very demanding in parameterssuch as resolution and overlay may be performed in a more advancedlithography tool than other layers that are less demanding. Thereforesome layers may be exposed in an immersion type lithography tool, whileothers are exposed in a ‘dry’ tool. Some layers may be exposed in a toolworking at DUV wavelengths, while others are exposed using EUVwavelength radiation.

In order that the substrates that are exposed by the lithographicapparatus are exposed correctly and consistently, it is desirable toinspect exposed substrates to measure properties such as overlay errorsbetween subsequent layers, line thicknesses, critical dimensions (CD),etc. Accordingly a manufacturing facility in which litho cell LC islocated also includes metrology system MET which receives some or all ofthe substrates W that have been processed in the litho cell. Metrologyresults are provided directly or indirectly to the supervisory controlsystem (SCS) 138. If errors are detected, adjustments may be made toexposures of subsequent substrates, especially if the metrology can bedone soon and fast enough that other substrates of the same batch arestill to be exposed. Also, already exposed substrates may be strippedand reworked to improve yield, or discarded, thereby avoiding performingfurther processing on substrates that are known to be faulty. In a casewhere only some target portions of a substrate are faulty, furtherexposures can be performed only on those target portions which are good.

Also shown in FIG. 1 is a metrology apparatus 140 which is provided formaking measurements of parameters of the products at desired stages inthe manufacturing process. A common example of a metrology apparatus ina modern lithographic production facility is a scatterometer, forexample an angle-resolved scatterometer or a spectroscopicscatterometer, and it may be applied to measure properties of thedeveloped substrates at 120 prior to etching in the apparatus 122. Usingmetrology apparatus 140, it may be determined, for example, thatimportant performance parameters such as overlay or critical dimension(CD) do not meet specified accuracy requirements in the developedresist. Prior to the etching step, the opportunity exists to strip thedeveloped resist and reprocess the substrates 120 through the lithocluster. As is also well known, the metrology results 142 from theapparatus 140 can be used to maintain accurate performance of thepatterning operations in the litho cluster, by supervisory controlsystem SCS and/or control unit LACU 106 making small adjustments overtime, thereby minimizing the risk of products being madeout-of-specification, and requiring re-work. Of course, metrologyapparatus 140 and/or other metrology apparatuses (not shown) can beapplied to measure properties of the processed substrates 132, 134, andincoming substrates 130.

Example Inspection Apparatus

FIG. 2(a) shows schematically the key elements of an inspectionapparatus implementing so-called dark field imaging metrology. Theapparatus may be a stand-alone device or incorporated in either thelithographic apparatus LA, e.g., at the measurement station, or thelithographic cell LC. An optical axis, which has several branchesthroughout the apparatus, is represented by a dotted line O. A targetgrating structure T and diffracted rays are illustrated in more detailin FIG. 2(b).

As described in the prior applications cited in the introduction, thedark-field-imaging apparatus of FIG. 2(a) may be part of a multi-purposeangle-resolved scatterometer that may be used instead of, or in additionto, a spectroscopic scatterometer. In this type of inspection apparatus,radiation emitted by a radiation source 11 is conditioned by anillumination system 12. For example, illumination system 12 may includea collimating lens system 12 a, a color filter 12 b, a polarizer 12 cand an aperture device 13. The conditioned radiation follows anillumination path IP, in which it is reflected by partially reflectingsurface 15 and focused into a spot S on substrate W via an objectivelens 16. A metrology target T may be formed on substrate W. Theobjective lens 16 may be similar in form to a microscope objective lens,but has a high numerical aperture (NA), preferably at least 0.9 and morepreferably at least 0.95. Immersion fluid can be used to obtainnumerical apertures over 1 if desired.

The objective lens 16 in this example serves also to collect radiationthat has been scattered by the target. Schematically, a collection pathCP is shown for this returning radiation. The multi-purposescatterometer may have two or more measurement branches in thecollection path. The illustrated example has a pupil imaging branchcomprising pupil imaging optical system 18 and pupil image sensor 19. Animaging branch is also shown, which will be described in more detailbelow. Additionally, further optical systems and branches will beincluded in a practical apparatus, for example to collect referenceradiation for intensity normalization, for coarse imaging of capturetargets, for focusing and so forth. Details of these can be found in theprior publications mentioned above.

Where a metrology target T is provided on substrate W, this may be a 1-Dgrating, which is printed such that, after development, the bars areformed of solid resist lines. The target may be a 2-D grating, which isprinted such that after development, the grating is formed of solidresist pillars or vias in the resist. The bars, pillars or vias mayalternatively be etched into the substrate. Each of these gratings is anexample of a target structure whose properties may be investigated usingthe inspection apparatus. In the case of gratings, the structure isperiodic. In the case of an overlay metrology target, the grating isprinted on top of or interleaved with another grating that has beenformed by a previous patterning step.

The various components of illumination system 12 can be adjustable toimplement different metrology ‘recipes’ within the same apparatus. Inaddition to selecting wavelength (color) and polarization ascharacteristics of the illuminating radiation, illumination system 12can be adjusted to implement different illumination profiles. The planeof aperture device 13 is conjugate with a pupil plane of objective lens16 and with the plane of the pupil image detector 19. Therefore, anillumination profile defined by aperture device 13 defines the angulardistribution of light incident on substrate W in spot S. To implementdifferent illumination profiles, an aperture device 13 can be providedin the illumination path. The aperture device may comprise differentapertures 13 a, 13 b, 13 c etc. mounted on a movable slide or wheel. Itmay alternatively comprise a fixed or programmable spatial lightmodulator (SLM). As a further alternative, optical fibers may bedisposed at different locations in the illumination pupil plane and usedselectively to deliver light or not deliver light at their respectivelocations. These variants are all discussed and exemplified in thedocuments cited above. The aperture device may be of a reflective form,rather than transmissive. For example, a reflective SLM might be used.Indeed, in an inspection apparatus working in the UV or EUV wavebandmost or all of the optical elements may be reflective.

Depending on the illumination mode, example rays 30 a may be provided sothat the angle of incidence is as shown at ‘I’ in FIG. 2(b). The path ofthe zero order ray reflected by target T is labeled ‘0’ (not to beconfused with optical axis ‘O’). Similarly, in the same illuminationmode or in a second illumination mode, rays 30 b can be provided, inwhich case the angles of incidence and reflection will be swappedcompared with the first mode. In FIG. 2(a), the zero order rays of thefirst and second example illumination modes are labeled 0 a and 0 brespectively.

As shown in more detail in FIG. 2(b), target grating T as an example ofa target structure is placed with substrate W normal to the optical axisO of objective lens 16. In the case of an off-axis illumination profile,a ray 30 a of illumination I impinging on grating T from an angle offthe axis O gives rise to a zeroth order ray (solid line O) and two firstorder rays (dot-chain line +1 and double dot-chain line −1). It shouldbe remembered that with an overfilled small target grating, these raysare just one of many parallel rays covering the area of the substrateincluding metrology target grating T and other features. Since the beamof illuminating rays 30 a has a finite width (necessary to admit auseful quantity of light), the incident rays I will in fact occupy arange of angles, and the diffracted rays 0 and +1/−1 will be spread outsomewhat. According to the point spread function of a small target, thediffracted radiation of each order +1 and −1 will be further spread overa range of angles, not a single ideal ray as shown.

In the branch of the collection path for dark-field imaging, imagingoptical system 20 forms an image T′ of the target on the substrate W onsensor 23 (e.g. a CCD or CMOS sensor). An aperture stop 21 is providedin a plane in the imaging branch of the collection path CP which isconjugate to a pupil plane of objective lens 16. Aperture stop 21 mayalso be called a pupil stop. Aperture stop 21 can take different forms,just as the illumination aperture can take different forms. The aperturestop 21, in combination with the effective aperture of lens 16,determines what portion of the scattered radiation is used to producethe image on sensor 23. Typically, aperture stop 21 functions to blockthe zeroth order diffracted beam so that the image of the target formedon sensor 23 is formed only from the first order beam(s). In an examplewhere both first order beams were combined to form an image, this wouldbe the so-called dark field image, equivalent to dark-field microscopy.

The images captured by sensor 23 are output to image processor andcontroller PU, the function of which will depend on the particular typeof measurements being performed. For the present purpose, measurementsof asymmetry of the target structure are performed. Asymmetrymeasurements can be combined with knowledge of the target structures toobtain measurements of performance parameters of lithographic processused to form them. Performance parameters that can be measured in thisway include for example overlay, focus and dose. Special designs oftargets are provided to allow these measurements of differentperformance parameters to be made through the same basic asymmetrymeasurement method.

Processor and controller PU also generates control signals such as λ andAP, for controlling the illumination characteristics (polarization,wavelength) and for selecting the aperture using aperture device 13 or aprogrammable spatial light modulator. Aperture stop 21 may also becontrolled in the same way. Each combination of these parameters of theillumination and the detection is considered a “recipe” for themeasurements to be made.

Referring again to FIG. 2(b) and the illuminating rays 30 a, +1 orderdiffracted rays from the target grating will enter the objective lens 16and contribute to the image recorded at sensor 23. Rays 30 b areincident at an angle opposite to rays 30 a, and so the −1 orderdiffracted rays enter the objective and contribute to the image.Aperture stop 21 blocks the zeroth order radiation when using off-axisillumination. As described in the prior publications, illumination modescan be defined with off-axis illumination in X and Y directions.

Apertures 13 c, 13 e and 13 f in the aperture device 13 of FIG. 2(a)include off-axis illumination in both X and Y directions, and are ofparticular interest for the present disclosure. Aperture 13 c createswhat may be referred to as a segmented illumination profile, and may forexample be used in combination with a segmented aperture defined forexample by a segmented prism 22, described below. Apertures 13 e and 13f may for example be used in combination with an on-axis aperture stop21, in a manner described in some the prior published patentapplications, mentioned above.

By comparing images of the target grating under these differentillumination modes, asymmetry measurements can be obtained.Alternatively, asymmetry measurements could be obtained by keeping thesame illumination mode, but rotating the target. While off-axisillumination is shown, on-axis illumination of the targets may insteadbe used and a modified, off-axis aperture stop 21 could be used to passsubstantially only one first order of diffracted light to the sensor. Ina further example, a segmented prism 22 is used in combination with anon-axis illumination mode. The segmented prism 22 can be regarded as acombination of individual off-axis prisms, and can be implemented as aset of prisms mounted together, if desired. These prisms define asegmented aperture in which rays in each quadrant are deflected slightlythrough an angle. This deflection in the pupil plane in has the effectof spatially separating the +1 and −1 orders in each direction in theimage plane. In other words, the radiation of each diffraction order anddirection forms an image to different locations on sensor 23 so thatthey can be detected and compared without the need for two sequentialimage capture steps. Effectively, separate images are formed atseparated locations on the image sensor 23. In FIG. 2(a) for example, animage T′(+1 a), made using +1 order diffraction from illuminating ray 30a, is spatially separated from an image T′(−1 b) made using −1 orderdiffraction from illuminating ray 30 b. This technique is disclosed inthe above-mentioned published patent application US20110102753A1, thecontents of which are hereby incorporated by reference in its entirety.2nd, 3rd and higher order beams (not shown in FIG. 2) can be used inmeasurements, instead of, or in addition to, the first order beams. As afurther variation, the off-axis illumination mode can be kept constant,while the target itself is rotated 180 degrees beneath objective lens 16to capture images using the opposite diffraction orders.

Whichever of these techniques is used, the present disclosure applies tomethods in which radiation diffracted in two directions, for example theorthogonal directions called X and Y, is simultaneously captured.

While a conventional lens-based imaging system is illustrated, thetechniques disclosed herein can be applied equally with plenopticcameras, and also with so-called “lensless” or “digital” imagingsystems. There is therefore a large degree of design choice, which partsof the processing system for the diffracted radiation are implemented inthe optical domain and which are implemented in the electronic andsoftware domains.

Image-Based Asymmetry Measurement

Referring to FIG. 3(a), and viewing the pupil plane of the illuminationsystem P(IP) in the vicinity of aperture device 13, aperture 13 c hasbeen selected to define a specific spatial profile of illumination,illustrated at 902. In this desired spatial profile of the illuminationsystem, two diametrically opposite quadrants, labeled a and b, arebright, while the other two quadrants are dark (opaque). This spatialillumination profile, when focused to form spot S on the target T,defines a corresponding angular distribution of illumination, in whichrays from angles only in these two quadrants. This segmented type ofaperture is known in scatterometry apparatus, from the published patentapplication US 2010/201963. The merits of this modified illuminationaperture will be described further below.

When rays from the bright segments of the illumination profile 902 arediffracted by periodic features in a target structure, they will be atangles corresponding to a shift in the pupil plane. Arrows ‘x’ in FIG. 3(a) indicate the direction of diffraction of illumination caused bystructures periodic in the X direction, while arrows ‘y’ indicate thedirection of diffraction of illumination caused by structures periodicin the Y direction. Arrows ‘0’ indicate direct reflection, in otherwords zero order diffraction. A feature of this segmented type ofaperture is that, with regard to lines of symmetry defined by expecteddirections of diffraction (X and Y in this example), illuminated regionsof the illumination profile are symmetrically opposite dark regions.Therefore there is the possibility to segregate the higher orderdiffracted radiation, while collecting radiation directed in bothdirections simultaneously.

FIG. 3(b) illustrates a distribution of illumination in a conjugatepupil plane P(CP) in the collection path of the inspection apparatus.Assume firstly that the target T is a one-dimensional diffractiongrating, with a periodicity in the X direction as a first direction.While the spatial profile 902 of the illumination has bright quadrantslabeled a and b, the diffraction pattern resulting from diffraction bythe lines of the target grating is represented by the pattern at 904 inFIG. 3(b). In this pattern, in addition to zero order reflectionslabeled a₀ and b₀ there are first order diffraction signals visible,labeled a_(+x), b_(−x). Because other quadrants of the illuminationaperture are dark, and more generally because the illumination patternhas 180° rotational symmetry, the diffraction orders a_(+x) and b_(−x)are “free” meaning that they do not overlap with the zero order orhigher order signals from other parts of the illumination aperture(considering only the X direction at this stage). This property of thesegmented illumination pattern can be exploited to obtain clear firstorder signals from a diffraction grating (alignment mark) having a pitchwhich is half the minimum pitch that could be imaged if a conventional,circularly-symmetric illumination aperture were used.

Now, assume that the target has periodic features in a second direction,for example the Y direction which is orthogonal to the first direction.These features in the second direction may arise from segmentation inthe nominally one-dimensional grating, they may also arise from otherone-dimensional gratings with Y orientation, that may be present withinthe area of spot S and the within the field of view of the inspectionapparatus. They may also arise from a mixture of these. Assume furtherthat the features periodic in the Y direction have the same period, andtherefore the same diffraction angle, as the features periodic in the Xdirection. The result is diffraction signals a_(+y) and b_(−y) that canbe seen in the pupil 904 of the collection path. These signals comprisefirst order diffraction signals in the Y direction. For simplicity ofillustration in the present drawings, the diffraction signals in the Ydirection and the X direction are shown as free of one another. Inpractice, the X diffraction signals and the Y diffraction may overlap inthe pupil 904. The reader skilled in the art will understand that thisdepends on the pitches of the target in X and Y and the chosenwavelength.

Zero order signals a₀ and b₀ are also present in the pupil of thecollection system, as illustrated. Depending whether these zero ordersignals are wanted or not, they may be blocked by a segmented aperturestop 21, similar in form to aperture 13 d. For asymmetry-basedmeasurements, it is generally the higher order signals, for example the+1 and −1 order signals, that are of interest.

As illustrated, the Y direction diffraction signals do not overlap the Xdirection diffraction signals in the pupil of the collection path, butin other situations they might overlap, depending on the pitch of thegrating and the wavelength of illumination. In the case of segmentedgratings, the segmentation in one or both directions may be much finerthan the pitch of the grating in the other direction. Where very finesegmentation is present, the higher order diffraction signals may falloutside the aperture of the collection path, but scattering of zeroorder radiation may spill into the quadrants at top left and bottomright. In any case, where two-dimensional features of some kind arepresent, diffraction signals from two directions are mixed in the samequadrants of the pupil in the collection path.

FIG. 3(c) shows schematically the layout of the segmented prism 22 inthe imaging branch of the inspection apparatus of FIG. 2. The circularpupil P(CP) is represented by a dotted circle. In each quadrant of thepupil, a differently angled prism is provided, which deflects theradiation through a certain angle. This angular deflection in the pupilplane translates into a spatial separation of images in the plane of thedetector 23, as illustrated already above with reference to FIG. 2(a).The operation of the apparatus in this type of configuration, and somepractical benefits and challenges, will now be described in further. Theprinciples of the present disclosure are applicable in otherconfigurations, however.

FIG. 4 depicts a composite target formed on a substrate W according toknown practice. The composite target comprises four gratings 32 to 35positioned closely together so that they will all be within themeasurement spot S formed by the illumination beam of the metrologyapparatus. A circle 31 indicates the extent of spot S on the substrateW. The four targets thus are all simultaneously illuminated andsimultaneously imaged on sensor 23. In an example dedicated to overlaymeasurement, gratings 32 to 35 are themselves overlay gratings formed byoverlying gratings that are patterned in different layers of thesemi-conductor device formed on substrate W. Gratings 32 to 35 may bedifferently biased, meaning that they have designed-in overlay offsetsadditional to any unknown overlay error introduced by the patterningprocess. Knowledge of the biases facilitates measurement of overlaybetween the layers in which the different parts of the overlay gratingsare formed. Gratings 32 to 35 may also differ in their orientation, asshown, so as to diffract incoming radiation in X and Y directions.

In one example, gratings 32 and 34 are X-direction gratings with biasesof +d, −d, respectively. This means that grating 32 has its overlyingcomponents arranged so that if they were both printed exactly at theirnominal locations one of the components would be offset relative to theother by a distance d. Grating 34 has its components arranged so that ifperfectly printed there would be an offset of d but in the oppositedirection to the first grating and so on. Gratings 33 and 35 areY-direction gratings with offsets +d and −d respectively. Separateimages of these gratings can be identified in the image captured bysensor 23. While four gratings are illustrated, another embodiment mightrequire a larger matrix to obtain the desired accuracy.

FIG. 5 shows an example of an image that may be formed on and detectedby the sensor 23, using the target of FIG. 4 in the apparatus of FIGS.2-3, using the segmented illumination profile and using the segmentedprisms 22. Such a configuration provides off-axis illumination in both Xand Y orientations simultaneously, and permits detection of diffractionorders in X and Y simultaneously, from the quadrants at upper left andlower right of the pupil 904 in FIG. 3(b).

The dark rectangle 40 represents the field of the image on the sensor,within which the illuminated spot 31 on the substrate is imaged intofour corresponding circular areas, each using radiation only from onequadrant of the pupil 904 in the collection path CP. Four images of thetarget are labelled 502 to 508. Within image 502 the image of theilluminated spot 31 using radiation of the upper left quadrant of thepupil 904 is labelled 41. Within this, rectangular areas 42-45 representthe images of the small target gratings 32 to 35. If the gratings arelocated in product areas, product features may also be visible in theperiphery of this image field. Image processor and controller PUprocesses these images using pattern recognition to identify theseparate images 42 to 45 of gratings 32 to 35. In this way, the imagesdo not have to be aligned very precisely at a specific location withinthe sensor frame, which greatly improves throughput of the measuringapparatus as a whole.

As mentioned and as illustrated, because of the action of the segmentedprism 22 on the signals in the pupil 904 of the collection path, andbecause of the segmented illumination profile 902 and its orientationrelative to the X and Y directions of the target T, each of the fourimages 502-508 uses only certain portions of the diffraction spectra ofeach target. Thus the images 504 and 508 at lower left and upper rightrespectively are formed of the zero order radiation a₀ and b₀respectively. The image 502 is formed of higher order diffractedradiation, specifically radiation diffracted in the negative X directionfrom bright quadrant b and the positive Y direction from bright quadranta (diffraction signals a_(+y) and b_(−x)). Conversely, image 506 isformed of higher order diffracted radiation, specifically radiationdiffracted in the positive X direction from bright quadrant b and thenegative Y direction from bright quadrant a (diffraction signals a_(−y)and b_(+x)).

From the target comprising only one-dimensional gratings, there is nocross-talk between signals diffracted in the X direction and signalsdiffracted in the Y direction. That is because each component grating31-35 diffracts radiation in only one of the two directions, and theimage of each grating is spatially separated within the images 502-508by the imaging action of the optical system. Once the separate images ofthe gratings have been identified, the intensities of those individualimages can be measured, e.g., by averaging or summing selected pixelintensity values within the identified areas (ROIs). Intensities and/orother properties of the images can be compared with one another toobtain measurements of asymmetry for the four or more gratingssimultaneously. These results can be combined with knowledge of thetarget structures and bias schemes, to measure different parameters ofthe lithographic process. Overlay performance is an important example ofsuch a parameter, and is a measure of the lateral alignment of twolithographic layers. Overlay can be defined more specifically, forexample, as the lateral position difference between the center of thetop of a bottom grating and the center of the bottom of a correspondingtop-grating. To obtain measurements of other parameters of thelithographic process, different target designs can be used. Again,knowledge of the target designs and bias schemes can be combined withasymmetry measurements to obtain measurements of the desired performanceparameter. Target designs are known, for example, for obtainingmeasurements of dose or focus from asymmetry measurements obtained inthis way.

Problems with Two-Dimensional Targets

Referring now to FIG. 4(b), as mentioned above, some targets willscatter or diffract radiation in two directions within the same part ofthe image. The target of FIG. 4(b) has two-dimensional structures ineach of the four component gratings 432-435. The two dimensionalstructures may arise from segmentation in a one-dimensional grating inone or more layers. The two-dimensional structures may arise fromgratings representing arrays of contact holes or vias, for example.

Although diffraction will therefore occur in both directions X and Y,within each grating image 42-45, nevertheless the purpose of themetrology target is to measure a parameter such as overlay separately ineach of the X and Y directions. The contribution of diffraction from theother direction, in the same part of the image, represents“contamination” or noise in the wanted diffraction signals. In theoverlay measurement we derive X-Overlay from the asymmetry (differencebetween +1st and −1st order diffraction) in the X direction. The addedradiation from diffraction in the Y direction leads to a worse signal tonoise ratio. If the segmentation is present in both layers (or has anasymmetric shape), the added diffraction will not just add light, butalso add asymmetry. This will lead to measurement errors, on top of thesignal to noise degradation.

In the case of a two-dimensional overlay grating, bias values can be setseparately in X and/or Y directions.

Discriminating Asymmetry of Diffraction in Different Directions

Referring now to FIG. 6, a method according to the present disclosureaddresses the problem identified above. Embodiments based on thisprinciple can bring one or both of the following benefits:

1) Prevent measurement errors due to the added diffraction from thesecond direction

2) Provide a means to detect the presence of the additional Y-signal, sothat extra measures can be taken when needed.

With regard to the second benefit, it was noted above that it may noteven be known a priori, whether a target has structure in two directionsor one.

The inventors have realized that the asymmetry created by diffraction ina first direction (e.g. the X direction) can be separated from thediffraction in a second direction (e.g. the Y direction) by combiningmeasurements made using two different angular distributions ofillumination, relative to the target structure. In particularembodiments, it is convenient to use two orientations of the sameillumination profile, which is the method illustrated in FIG. 6. At theleft hand side in FIG. 6, parts (a), (c), (e) illustrate theconfiguration shown in FIG. 3, while at the right had side parts (b),(d), (f) illustrates a second configuration which implements a similarmeasurement but uses a second angular illumination profile. Using thesetwo angular illumination profiles in combination, the signals caused bydiffraction in the different directions can be discriminated. In theexample of FIG. 6, these different angular illumination profiles areimplemented by different rotational positions of the substrate, relativeto the optical system of the inspection apparatus.

FIG. 6(a) shows a first orientation of substrate W and target T (zerodegrees, suffix ‘0’), while (b) shows a second orientation of substrateW and target T (ninety degrees, suffix ‘R’). FIGS. 6(c) and (d) showshow the resulting illumination profiles 902(0) and 902(R) are the samerelative to the optical system, but are rotated relative to thedirections X and Y defined by the target structure. FIGS. 6(e) and (f)show how the distribution of radiation in each quadrant of the pupils904(0) and 904(R) is therefore similar, but contains different orders ofdiffraction. From images detected using these two angular illuminationprofiles, the signals caused by diffraction in the different directionscan be discriminated.

FIG. 7 shows the images 740(0) and 740(R) obtained by the two imagecapture steps illustrated in FIG. 6. Image 740(0) is the same as thatshown in FIG. 5, with four spatially separated images 702(0)-708(0) ofthe target. As described already for FIG. 5, the image 702(0) is formedof radiation diffracted by the target in the negative X direction andthe positive Y direction (labelled −x/+y). Image 706(0) is formed ofradiation diffracted in the positive X direction and the negative Ydirection (+x/−y). These images 702(0)-708(0) will (for the same targetand measurement conditions) be the same as images 502-508 shown in FIG.5. On the other hand, due to the different orientation of the target Tin capturing image 740(R), images 702(R) to 708(R) within image 740(R)will have different combinations of diffraction orders. With the axesand labelling conventions adopted above, the image 702(R) will be formedof radiation diffracted by the target in the negative X direction andthe negative Y direction (labelled −x/−y). Image 706(R) will be formedof radiation diffracted in the positive X direction and the positive Ydirection (+x/+y).

In the case where these images are obtained by rotating the targetstructure under the same illumination profile, the arrangement of thedifferent parts of the target structure will also be rotated within theimages 702(R) to 708(R), compared with the arrangement in images 702(0)to 708(0). This rotation can be taken into account when selecting thepixels and ROIs to combine their intensities. To illustrate this, a dotin each image indicates the part corresponding to the first componentgrating 32.

Now, if we consider how the intensities of the individual grating areaswithin the image are conventionally used to calculate overlay OV from apair of biased gratings, we use the formula:

$\begin{matrix}{{OV} = {{atan}\; \left( {\frac{A_{+ d} + A_{- d}}{A_{+ d} - A_{- d}} \cdot {\tan (d)}} \right)}} & (1)\end{matrix}$

where A_(+d) is asymmetry measured between the intensity of oppositediffraction order images of a component grating with bias +d and A_(−d)is asymmetry measured between opposite diffraction order images of acomponent grating with bias −d. In Equation (1), the offset d isexpressed as an angle, relative to 2π radians representing the period ofthe grating.

If each grating is only one-dimensional, as in FIG. 3(a), then a singlecaptured image 40 as shown in FIG. 5 has the complete informationrequired to obtain independent measurements of overlay OV with respectto the X and Y directions. In the case where a grating in the target hastwo-dimensional structure, however, the diffraction signals fordifferent directions become mixed as described above. Fortunately, bycapturing the two images 740(0) and 740(R), the signals for the twodirections can be separated by a simple calculation. What is wanted isto obtain the directional asymmetry values A_(x) and A_(y), which arethe differences in the intensities of the +1 and −1 diffraction ordersonly for the X direction and only for the Y direction, respectively.

For any and each of the component gratings 432-435, four higher orderdiffraction intensities are available:

I_(−x/+y) from the image 702(0)

I_(+x/−y) from the image 706(0)

I_(−x/−y) from the image 702(R) and

I_(+x/+y) from the image 706(R).

Although the different directional asymmetries are mixed in each of thecaptured images, the nature of the mixing is slightly different. Bycombining information from the two images 740(0) and 740(R), theseparate contributions for each direction, specifically the desireddirectional asymmetry values A_(x) and A_(y), can be separated. Definingasymmetries A(0) and A(R) as asymmetries measured from the two imagesand combining them with the definitions of the desired directionalasymmetry values A_(x) and A_(y) we obtain:

A(0)=I _(−x/+y) −I _(+x/−y) =−A _(x) +A _(y)   (2)

A(R)=I _(+x/+y) −L _(−x/−y) =A _(x) +A _(y)   (3)

From this, the directional asymmetry values can be recovered usingformulae:

A _(x) =−A(0)+A(R)   (4)

A _(y) =A(0)+A(R)   (5)

In this way, the asymmetry of diffraction orders in a first direction(for example the X direction) is discriminated from the asymmetry ofdiffraction orders in the second direction (Y). Using these directionalasymmetry values for differently biased gratings in the formula ofEquation (1), overlay values specific to each direction can be obtainedfor the gratings of interest. Separate directional overlay values canalso be obtained for two-dimensional gratings, if the biases in X and Yare separately known, as explained in the application examples below.

Also, from this information, the two-dimensional character of a gratingcan be identified, where it may not have been known before. For example,based on the observation of a significant asymmetry signal A_(y) for agrating which is nominally an X direction grating, an inference can bemade that this grating has segmentation in the Y direction. The presenceor absence of two-dimensional features in an individual target or agroup or class of targets can be decided, for example, based on athreshold of asymmetry signals observed in the second direction.Strictly speaking, if a grating has segmentation in the Y direction, butis perfectly symmetrical in the Y direction, then this would not bedetected. However, in that hypothetical case, it would also not causeany error in the measurement of asymmetry-related parameters in the Xdirection, and so its two-dimensional character would be of academicinterest only.

In all the above equations, some scaling factors and normalizationfactors are omitted for simplicity. For example, as described in some ofthe prior published applications mentioned above, it may be convenientto normalize the differences between intensities using the average ofthose intensities as a denominator. So, for example, where above iswritten:

A(0)=I _(−x/+y) −I _(+x/−y) =−A _(x) +A _(y)   (2)

the full expression might be:

A(0)=2(I _(−x/+y) −I _(+x/−y))/(I _(−x/+y) +I _(+x/−y))=−A _(x) +A _(y)  (2′)

The person skilled in the art can incorporate these practical detailswith routine skill and knowledge.

Application Examples

FIG. 8 illustrates a method of measuring performance of a lithographicprocess using the apparatus and methods outlined above. In step S20, oneor more substrates are processed to produce target structures such asthe composite grating targets described above. The design of target canbe any of the known designs, such as those shown in FIG. 4(a) or (b) ornew designs, examples of which are described below. Targets may be largetarget or small target designs, depending whether the first measurementbranch or second measurement branch of the apparatus is to be used.Targets may be composite targets with distinct periodic structures indistinct areas. Targets may be designed for measurement of overlay,focus or dose through asymmetry. Targets may be designed for measurementof other performance parameters and/or non-asymmetry-related parameters.Linewidth or critical dimension CD is an example of a parameter that maybe measured by scatterometry other than through measurement ofasymmetry.

At step S20, structures are produced across a substrate using thelithographic manufacturing system, and the substrate is loaded into aninspection apparatus, such as the inspection apparatus of FIG. 2. Instep S21 metrology recipes are defined, including a recipe formeasurement using two or more illumination profiles, such as the rotatedprofiles as described above with reference to FIG. 6. All the usualparameters of such a recipe are also defined, including thepolarization, angular distribution and so forth. In other embodiments,more than two different angular distributions of illuminating radiation(illumination profiles) may be defined. As already mentioned, theillumination profiles are different relative to the directions definedby the target orientation. Therefore the recipes may in practice specifydifferent orientations of the substrate relative to the optical system,rather than changing the illumination profile within the optical system.Alternatively, for example, the segmented aperture 13 c may be rotatablethrough ninety degrees or two rotated versions of it may be provided forselection in the aperture device 13.

In step S22, the inspection apparatus is operated to capture two or moredark-field images (such as images 740(0) and 740(R) in FIG. 7) using thespecified illumination profiles. Properties such as asymmetry valuesA(0) and A(R) are calculated from the captured images of one or moretargets.

At step S22 a one or more directional asymmetry values A_(x) and A_(y)are calculated for one or more target structures, by selecting and/orcombining signals from the two or more dark-field images. Thesedirectional asymmetry values can be used to calculate one or moreparameters of interest relating to the target structures and/or relatingto the performance of the lithographic process by which the targetstructures have been formed. Parameters of interest include directionaloverlay values, and values for dose and focus, for example. Bydiscriminating between radiation diffracted in a direction of interestand radiation diffracted in a different, non-parallel direction, noiseis reduced in the asymmetry measurements, leading to more accuratemeasurements of a performance parameter such as overlay, focus and/ordose. Parameters of interest may be simply whether the image of a targetstructure contains a mixture of radiation diffracted in two directionsor not.

At step S23, the metrology recipe may be updated in response to theobtained measurements and ancillary data. For example, the metrologytechniques for a new product or target layout may be under development.Information about the two-dimensional characteristics can be used toselect the appropriate recipe. The recipe may not need an image to becaptured with different illumination profiles, if radiation diffractedin a second direction is found to be absent or insignificant. Even ifsignificant structure is present in the second direction, a recipe withdifferent wavelength and/or polarization may minimize the amount ofradiation diffracted in the second direction that gets mixed withdiffraction in the second direction.

In step S24, in a development and/or production phase of operating thelithographic production facility of FIG. 1, recipes for the lithographicprocess may be updated, for example to improve overlay in futuresubstrates. The ability to discriminate between radiation diffracted indifferent directions allows the efficient measurement techniques usingsegmented detection systems to be applied even when target structureshave significant two-dimensional structure. An inspection apparatus canbe used with a fixed, segmented detection system, while covering a fullrange of targets, reducing cost and size of the apparatus.

The calculations to obtain measurements, and to control the selection ofwavelengths and other recipe parameters, can be performed within theimage processor and controller PU of the inspection apparatus. Inalternative embodiments, the calculations of asymmetry and otherparameters of interest can be performed remotely from the inspectionapparatus hardware and controller PU. They may be performed for examplein a processor within supervisory control system SCS, or in any computerapparatus that is arranged to receive the measurement data from theprocessor and controller PU of the inspection apparatus. Control andprocessing of the calibration measurements can be performed in aprocessor separate from that which performs high-volume calculationsusing the correction values obtained. All of these options are a matterof choice for the implementer, and do not alter the principles appliedor the benefits obtained. Use of the term “processor” in the descriptionand claims should be understood also to encompass a system ofprocessors.

Alternative Target Layouts

As mentioned above, a particular goal of this disclosure as presentedabove is to allow for directional measurements of overlay and the likeon segmented targets. However, the directional discriminating power ofthe disclosed techniques can also be applied to obtain performanceparameters such as overlay in two different directions from the sametarget structure. A target with a two-dimensional grating pattern can becombined with known biases in both directions, and an asymmetry-relatedparameter such overlay can be determined separately in both directions.

As illustrated in FIG. 9, an advantage of such a target and detectionmethod can be that it needs only three gratings instead of four gratingsto measure overlay in both directions. (Alternatively, instead ofneeding nine gratings to accommodate 3 bias values in each of twodirections, only six might be needed.) Since metrology overhead is veryexpensive in “real estate” as well as in measurement time, such a savingin target area could be very advantageous.

In FIG. 9(a), an illumination spot 931 is shown illuminating a compositetarget is shown having three overlay gratings, 932-934. Two or more ofthese gratings are provided with different overlay bias values in afirst direction and two or more of these gratings are provided withdifferent overlay bias values in a second direction. As a first examplesof bias combinations that may be used, the two-dimensional bias (dx, dy)may be as follows for the three targets:

$\begin{matrix}{\left( {{dx},{dy}} \right) = \left( {{{+ 20}\mspace{14mu} {nm}},{{+ 20}\mspace{14mu} {nm}}} \right)} \\{\left( {{{- 20}\mspace{14mu} {nm}},{{+ 20}\mspace{14mu} {nm}}} \right)} \\{\left( {{{+ 20}\mspace{14mu} {nm}},{{- 20}\mspace{14mu} {nm}}} \right)}\end{matrix}\quad$

It will be seen that, using the method of FIGS. 6 to 8 to discriminatebetween asymmetry in different directions, overlay measurementsindependent in X and Y can be obtained from only three gratings, wherethe prior techniques require four gratings, as shown in FIG. 4.

Another example bias scheme is:

$\begin{matrix}{\left( {{dx},{dy}} \right) = \left( {{{+ 20}\mspace{14mu} {nm}},{{+ 20}\mspace{14mu} {nm}}} \right)} \\{\left( {{{- 20}\mspace{14mu} {nm}},{{+ 20}\mspace{14mu} {nm}}} \right)} \\{\left( {{0\mspace{14mu} {nm}},{{- 20}\mspace{14mu} {nm}}} \right)}\end{matrix}\quad$

Several other combinations are also possible. As long as the 3 points(dx, dy) are not on a straight line, then the overlay in the twodirections can be independently measured.

In some applications, symmetry signals (e.g. average intensity ofopposite diffraction orders) are used in addition to asymmetry signals,for example to characterize the target in some way. Using the abovereduced number of targets, these symmetry signals cannot be separatedbetween X and Y. For this reason, it may be preferred to have equalpitch of the periodic structures in X and Y directions, so that thosesymmetric signals are roughly equal for X and Y.

In some applications, it can be useful to measure asymmetry in twodirections on a single grating structure that is not an overlay grating.Targets designed for measurement of dose or focus, for example, may bedesigned to exhibit asymmetry in a single layer grating. As anotherexample of a single layer structure for which asymmetry is of interest,it is sometimes desirable to form an auxiliary target structurealongside a set of biased overlay gratings, which is identical in thebottom layer, but has no overlying top grating. This structure can beused to measure directly the bottom grating asymmetry (BGA) which couldotherwise become confused with a genuine overlay error. The technique ofFIGS. 6-8 can be used to measure the asymmetry of this bottom gratingstructure independently in both directions. Using the measured BGA, theoverlay measurements based on asymmetry can be corrected to moreaccurately reflect the true overlay error between the top and bottomgratings. In principle, top grating asymmetry could be measured in thesame way, but the more common problem is that of asymmetry introduced inthe bottom grating, which may be subjected to several chemical andphysical processing steps, prior to formation of the top grating.

FIG. 9(b) illustrates a composite target the same as that shown in FIG.9(a), but with a fourth target structure 935 occupying the fourthquarter of an overall square outline. Such a fourth structure may forexample be the bottom grating asymmetry target mentioned above, or someother auxiliary target structure. In this way, where adding an auxiliarytarget structure conventionally incurs an additional space penalty, theauxiliary target structure can be added to a three-grating target andonly occupies the same amount of space as the known targets.

The method of FIGS. 6-8 can also be used for example to measure a targetconsisting of a line grating in the bottom layer periodic in a firstdirection (e.g. X) and another line grating in the top layer that isperiodic in a second direction (e.g. Y). This would allow the apparatusto measure the bottom grating asymmetry (BGA) in X and the top gratingasymmetry in Y both from the same piece of target area.

Finally, as mentioned above, while the above techniques can be used tomeasure a property of the target independently in two directions, it mayalso be used as a simple check to see if segmentation or other structureis present in a second direction. If not, then the normal singleorientation measurement can be reliably executed. If the segmentationsignal is present in the second direction, the measurement for that typeof target can be subsequently executed in the mode described above or byalternative methods.

CONCLUSION

The principles disclosed above allow directional measurement accuracy tobe maintained when target structures having strong two-dimensionalcharacteristics. The technique is suitable for application in asymmetrymeasurements to be made by dark field imaging methods, using segmenteddetection systems, as well as other methods. Use of two or more angulardistributions of illumination allows the simple and efficient inspectionapparatus based on a segmented detection system to operate with a fullrange of targets, including those having significant diffraction in asecond direction.

Additionally, the disclosed method an apparatus can determine propertiesin two directions independently with a reduced number of biased targetsand hence with a saving in space on the substrate.

Additionally, the disclosed method and apparatus can deliver informationabout the previously—unknown two-dimensional character of the targetstructures. Such information may allow information about the performanceof the lithographic process to be derived, or at least it may allowselection of appropriate recipes.

While specific embodiments of the invention have been described above,it will be appreciated that the invention may be practiced otherwisethan as described.

While the inspection apparatus or tool illustrated in the embodimentscomprises a particular form of scatterometer having first and secondbranches for simultaneous imaging of pupil plane and substrate plane byparallel image sensors, alternative arrangements are possible. Ratherthan provide two branches permanently coupled to objective lens 16 withbeam splitter 17, the branches could be coupled selectively by a movableoptical element such as a mirror. The optical system could be madehaving a single image sensor, the optical path to the sensor beingreconfigured by movable elements to serve as a pupil plane image sensorand then a substrate plane image sensor.

While the optical system illustrated in FIG. 2 comprises refractiveelements, reflective optics can be used instead. For example the use ofreflective optics may enable the use of shorter wavelengths ofradiation.

While the target structures described above are metrology targetsspecifically designed and formed for the purposes of measurement, inother embodiments, properties may be measured on targets which arefunctional parts of devices formed on the substrate. Many devices haveregular, grating-like structures. The terms ‘target grating’ and ‘targetstructure’ as used herein do not require that the structure has beenprovided specifically for the measurement being performed.

In association with the inspection apparatus hardware and suitableperiodic structures realized on substrates and patterning devices, anembodiment may include a computer program containing one or moresequences of machine-readable instructions implementing methods ofmeasurement of the type illustrated above to obtain information about atarget structure and/or about a lithographic process. This computerprogram may be executed, for example, within image processor andcontroller PU in the apparatus of FIG. 2 and/or the control unit LACU ofFIG. 1. There may also be provided a data storage medium (e.g.,semiconductor memory, magnetic or optical disk) having such a computerprogram stored therein.

Although specific reference may have been made above to the use ofembodiments of the invention in the context of optical lithography, itwill be appreciated that the invention may be used in otherapplications, for example imprint lithography, and where the contextallows, is not limited to optical lithography. In imprint lithography atopography in a patterning device defines the pattern created on asubstrate. The topography of the patterning device may be pressed into alayer of resist supplied to the substrate whereupon the resist is curedby applying electromagnetic radiation, heat, pressure or a combinationthereof. The patterning device is moved out of the resist leaving apattern in it after the resist is cured.

Further embodiments according to the invention are described in belownumbered clauses:

1. A method of determining a property of at least a first part of atarget structure formed by a lithographic process, the method beingbased on radiation diffracted by periodic features within the targetstructure and including the following steps:

-   -   (a) using a detection system to form a first image of the target        structure when illuminated with radiation having a first angular        distribution, the first image being formed using a selected        portion of radiation diffracted by the target structure in a        first direction and a selected portion of radiation diffracted        by the target structure in a second direction, said first and        second directions being defined relative to the target structure        and being non-parallel;    -   (b) using the detection system to form a second image of the        target structure when illuminated with radiation having a second        angular illumination distribution, the first and second angular        illumination profiles being oriented differently to one another,        relative to the target structure;

(c) combining intensity values from the first image and the second imageso as to discriminate between radiation diffracted in the firstdirection by a first part of the target structure and radiationdiffracted in the second direction by the same first part of the targetstructure; and

-   -   (d) based at least partly on the discrimination made in step        (c), determining the property of said first part of the        structure.

2. A method according to clause 1 wherein, based on the discriminationin step (c), step (d) determines whether the first part of the targetstructure is configured to cause significant asymmetrical diffraction inthe second direction.

3. A method according to clause 1 wherein, based on the discriminationin step (c), step (d) determines said property of the first part of thetarget structure based on the radiation diffracted in the firstdirection by the first part of the target structure and disregarding theeffect of the radiation diffracted in the second direction by the samefirst part of the target structure.

4. A method according to clause 3 wherein said property is related toasymmetry of the first part of the target structure in the firstdirection.

5. A method according to clause 4 wherein said property related toasymmetry of the part of the target structure in the first direction iscalculated based at least partly on a difference of intensity betweencomplementary portions of said first image and complementary portions ofsaid second image, the complementary portions in each image being imagesof the first part of the target structure formed using oppositediffraction orders of the radiation diffracted in the first direction.

6. A method according to clause 5 further comprising determining saidproperty related to asymmetry for at least a second part of the targetstructure and calculating a measurement of a first performance parameterof the lithographic process based on the property determined for thefirst part and the second part of the target structure and based onknown bias properties of the first part and the second part of thetarget structure in the first direction.

7. A method according to clause 5 or 6 further comprising determining asecond property of the first part of the target structure, the secondproperty being related to asymmetry in the second direction.

8. A method according to clause 7 further comprising determining saidsecond property related to asymmetry of at least a second part of thetarget structure in the second direction and calculating a measurementof a second performance parameter of the lithographic process based onthe properties determined for the first part and the second part of thetarget structure and based on known bias properties of thefirst-mentioned part and the second part of the target structure in thesecond direction.

9. A method according to clause 8 further comprising a step:

-   -   (e) combining intensity values from the first image and the        second image so as to discriminate between radiation diffracted        in the second direction by a third part and a fourth part of the        target structure and radiation diffracted in the first direction        by the same third part and fourth part of the target structure;        and    -   (f) based on the discrimination made in step (e), calculating        properties of said third and fourth parts of the target        structure related to asymmetry of the third and fourth parts of        the target structure in the second direction, and calculating a        measurement of a third performance parameter of the lithographic        process based on the properties determined for the third and        fourth parts of the target structure and based on known bias        properties of the third and fourth parts of the target        structure.

10. A method according to any preceding clause wherein said firstangular distribution of radiation is derived from a segmentedillumination profile having illuminated regions and dark regions, eachilluminated region being symmetrically opposite a dark region, whenreflected in the first direction and when reflected in the seconddirection.

11. A method according to clause 10 wherein said segmented illuminationprofile has four quadrants, said illuminated regions falling only withintwo quadrants diametrically opposite one another.

12. A method according to clause 10 or 11 wherein said detection systemis a segmented detection system, whereby each of said first image andsaid second image, includes complementary portions which are images ofthe target structure structures formed using opposite diffraction ordersof the radiation diffracted by the target structure.

13. A method according to any preceding clause wherein said targetstructure comprises two or more parts each with different programmedbias values only in said first direction and two or more parts each withdifferent programmed bias values only in said second direction.

14. A method according to any of clauses 1 to 12 wherein said targetstructure comprises three or more parts each periodic in both the firstdirection and the second direction, the different parts having differentcombinations of programmed bias values in the first direction and in thesecond direction.

15. A method according to any of clauses 1 to 12 wherein said targetstructure includes three parts each periodic in both the first directionand the second direction, at least two parts among said three partshaving different programmed bias values in the first direction and atleast two parts among said three parts having different programmed biasvalues in the second direction.

16. A method clause 15 wherein each of said three parts is an overlaygrating comprising grating structures formed in two or more layers andwherein said target structure further comprises a fourth part comprisinga grating structure formed in only one of said layers.

17. A method according to clause 16 wherein said target structure has arectangular layout divided into similar quarters, wherein said threeparts are arranged in three quarters of the rectangular layout and saidfourth part is arranged in a fourth quarter.

18. A method according to clause 17 wherein said rectangular layout is asubstantially square layout and said quarters are generally square.

19. A method according to any preceding clause further comprising usingthe determined property to modify a metrology recipe for measuringfurther target structures.

20. A method according to any preceding clause further comprising usingthe determined property to control a lithographic apparatus to applypatterns to substrates.

21. An inspection apparatus for measuring a property of a targetstructure formed by a lithographic process on one or more substrates,the inspection apparatus comprising:

-   -   an illumination system operable to illuminate a target structure        at different times with radiation having a first angular        distribution and a second angular distribution;    -   a detection system operable to form one or more images of the        target structure using selected portions of radiation diffracted        by the target structure; a controller for controlling the        illumination system and the detection system (a) to form a first        image of the target structure when illuminated with radiation        having a first angular distribution, the first image being        formed using a selected portion of radiation diffracted by the        target structure in a first direction and a selected portion of        radiation diffracted by the target structure in a second        direction, said first and second directions being defined        relative to the target structure and being non-parallel and (b)        to form a second image of the target structure when illuminated        with radiation having a second angular illumination        distribution, the first and second angular illumination profiles        being oriented differently to one another, relative to the        target structure.

22. An inspection apparatus according to clause 21 further comprising aprocessor configured (c) to combine intensity values from the firstimage and the second image so as to discriminate between radiationdiffracted in the first direction by a first part of the targetstructure and radiation diffracted in the second direction by the samefirst part of the target structure and (d) based at least partly on thediscrimination made in step (c), to the property of said first part ofthe structure.

23. An inspection apparatus according to clause 22 wherein said propertyis related to asymmetry of the first part of the target structure in thefirst direction.

24. An inspection apparatus according to clause 23 wherein saidprocessor is configured to calculate said property related to asymmetryof the part of the target structure in the first direction based atleast partly on a difference of intensity between complementary portionsof said first image and complementary portions of said second image, thecomplementary portions in each image being images of the first part ofthe target structure formed using opposite diffraction orders of theradiation diffracted in the first direction.

25. An inspection apparatus according to clause 24 wherein saidprocessor is further configured to calculate said property related toasymmetry for at least a second part of the target structure andcalculating a measurement of a first performance parameter of thelithographic process based on the property determined for the first partand the second part of the target structure and based on known biasproperties of the first part and the second part of the target structurein the first direction.

26. An inspection apparatus according to clause 24 or 25 wherein saidprocessor is further configured to determine a second property of thefirst part of the target structure, the second property being related toasymmetry in the second direction.

27. An inspection apparatus according to clause 26 wherein saidprocessor is further configured to determine said second propertyrelated to asymmetry of at least a second part of the target structurein the second direction and to calculate a measurement of a secondperformance parameter of the lithographic process based on theproperties determined for the first part and the second part of thetarget structure and based on known bias properties of thefirst-mentioned part and the second part of the target structure in thesecond direction.

28. An inspection apparatus according to clause 27 wherein saidprocessor is further configured (e) to combine intensity values from thefirst image and the second image so as to discriminate between radiationdiffracted in the second direction by a third part and a fourth part ofthe target structure and radiation diffracted in the first direction bythe same third part and fourth part of the target structure and (f)based on the discrimination made in step (e), to calculate properties ofsaid third and fourth parts of the target structure related to asymmetryof the third and fourth parts of the target structure in the seconddirection, and to calculate a measurement of a third performanceparameter of the lithographic process based on the properties determinedfor the third and fourth parts of the target structure and based onknown bias properties of the third and fourth parts of the targetstructure.

29. An inspection apparatus according to any of clauses 21 to 28 whereinsaid illumination system is operable to form said first angulardistribution of radiation using a segmented illumination profile havingilluminated regions and dark regions, each illuminated region beingsymmetrically opposite a dark region, when reflected in the firstdirection and when reflected in the second direction.

30. An inspection apparatus according to clause 29 wherein saidsegmented illumination profile has four quadrants, said illuminatedregions falling only within two quadrants diametrically opposite oneanother.

31. An inspection apparatus according to clause 29 or 30 wherein saiddetection system is a segmented detection system, whereby each of saidfirst image and said second image, includes complementary portions whichare images of the target structure structures formed using oppositediffraction orders of the radiation diffracted by the target structure.

32. An inspection apparatus according to clause 31 wherein saidsegmented detection system includes a segmented prism in a pupil planeof the detection system for deflecting said opposite diffraction ordersthrough different angles, thereby to form said complementary portionswith a spatial separation in an image plane of the detection system.

33. A target structure for use in a method according to any of clauses 1to 20 wherein said target structure includes at least three parts eachperiodic in both a first direction and a second direction, the first andsecond directions being non-parallel, at least two parts among saidthree parts having different programmed bias values in the firstdirection and at least two parts among said three parts having differentprogrammed bias values in the second direction.

34. A target structure according to clause 33 wherein each of said threeparts is an overlay grating comprising grating structures formed in twoor more layers and wherein said target structure further comprises afourth part comprising a grating structure formed in only one of saidlayers.

35. A target structure according to clause 34 wherein said targetstructure has a rectangular layout divided into similar quarters,wherein said three parts are arranged in three quarters of therectangular layout and said fourth part is arranged in a fourth quarter.

36. A target structure according to clause 35 wherein said rectangularlayout is a substantially square layout and said quarters are generallysquare.

37. A processing device arranged to receive at least first and secondimages of a target structure and to derive a measurement of one or moreproperties of one or more parts of a target structure by performing thesteps (c) and (d) in the method of any of clauses 1 to 20.

38. A computer program product comprising machine readable instructionsfor causing a programmable processing device to receive at least firstand second images of a target structure and to derive a measurement ofone or more properties of one or more parts of the target structure byperforming the steps (c) and (d) in the method of any of clauses 1 to20.

39. A computer program product according to clause wherein said machinereadable instructions are further arranged to cause the programmableprocessing device to control automatically the operation of aninspection apparatus to cause capture of the first and second images bysteps (a) and (b) of the method.

40. A lithographic system comprising:

-   -   a lithographic apparatus for applying a pattern onto one or more        substrates;    -   an inspection apparatus according to any of clauses 21 to 32;        and    -   a control system for controlling the lithographic apparatus        using the measurement results from the inspection apparatus,        when applying the pattern to further substrates.

41. A method of manufacturing devices wherein a device pattern isapplied to a series of substrates using a lithographic process, themethod including measuring one or more properties of at one or morestructures formed as part of or beside said device pattern on at leastone of said substrates using a method according to any of clauses 1 to20, and controlling the lithographic process for later substrates inaccordance with the result of the measuring.

The terms “radiation” and “beam” used herein encompass all types ofelectromagnetic radiation, including ultraviolet (UV) radiation (e.g.,having a wavelength of or about 365, 355, 248, 193, 157 or 126 nm) andextreme ultra-violet (EUV) radiation (e.g., having a wavelength in therange of 1-100 nm), as well as particle beams, such as ion beams orelectron beams. Implementations of scatterometers and other inspectionapparatus can be made in UV and EUV wavelengths using suitable sources,and the present disclosure is in no way limited to systems using IR andvisible radiation.

The term “lens”, where the context allows, may refer to any one orcombination of various types of optical components, includingrefractive, reflective, magnetic, electromagnetic and electrostaticoptical components. Reflective components are likely to be used in anapparatus operating in the UV and/or EUV ranges.

The breadth and scope of the present invention should not be limited byany of the above-described exemplary embodiments, but should be definedonly in accordance with the following claims and their equivalents.

1. An inspection apparatus for measuring a property of a targetstructure formed by a lithographic process on one or more substrates,the inspection apparatus comprising: an illumination system configuredto illuminate a target structure at different times with radiationhaving a first angular distribution and a second angular distribution; adetection system configured to form one or more images of the targetstructure using selected portions of radiation diffracted by the targetstructure; and a controller configured to control the illuminationsystem and the detection system to: form a first image of the targetstructure when illuminated with radiation having a first angularillumination distribution, the first image being formed using a selectedportion of radiation diffracted by the target structure in a firstdirection and a selected portion of radiation diffracted by the targetstructure in a second direction, the first and second directions beingdefined relative to the target structure and being non-parallel, andform a second image of the target structure when illuminated withradiation having a second angular illumination distribution, the firstand second angular illumination profiles being oriented differently toone another, relative to the target structure.
 2. The inspectionapparatus of claim 1, further comprising a processor coupled to thedetection system and the controller, the processor configured todetermine a property of the target structure based on a discriminationbetween the first and second images.
 3. The inspection apparatus ofclaim 2, wherein the processor is configured to determine a firstproperty of a first part of the target structure based on adiscrimination between radiation diffracted in the first direction bythe first part of the target structure and radiation diffracted in thesecond direction by the first part of the target structure.
 4. Theinspection apparatus of claim 3, wherein the first property is based onasymmetry of the first part of the target structure in the firstdirection.
 5. The inspection apparatus of claim 3, wherein the processoris configured to determine a second property of the first part of thetarget structure based on a discrimination between radiation diffractedin the first direction by the first part of the target structure andradiation diffracted in the second direction by the first part of thetarget structure.
 6. The inspection apparatus of claim 5, wherein thesecond property is based on asymmetry of the first part of the targetstructure in the second direction.
 7. The inspection apparatus of claim2, wherein the processor is configured to measure a first performanceparameter of the lithographic process based on the property of thetarget structure and bias values of the target structure in the firstdirection.
 8. The inspection apparatus of claim 7, wherein the processoris configured to measure a second performance parameter of thelithographic process based on the property of the target structure andbias values of the target structure in the second direction.
 9. Theinspection apparatus of claim 1, wherein the target structure comprisesat least three parts each periodic in both a first direction and asecond direction, the first and second directions being non-parallel.10. The inspection apparatus of claim 9, wherein at least two partsamong the three parts of the target structure comprise different biasvalues in the first direction.
 11. The inspection apparatus of claim 10,wherein at least two parts among the three parts of the target structurecomprise different bias values in the second direction.
 12. A targetstructure comprising: at least three parts each periodic in both a firstdirection and a second direction, the first and second directions beingnon-parallel; at least two parts among the three parts having differentbias values in the first direction; and at least two parts among thethree parts having different bias values in the second direction. 13.The target structure of claim 12, wherein each of the three parts is anoverlay grating comprising grating structures formed in two or morelayers.
 14. The target structure of claim 13, wherein the targetstructure further comprises a fourth part comprising a grating structureformed in only one of the layers.
 15. The target structure of claim 14,wherein the target structure comprises a rectangular layout divided intosimilar quarters.
 16. The target structure of claim 15, wherein thethree parts are arranged in three quarters of the rectangular layout andthe fourth part is arranged in a fourth quarter.
 17. The targetstructure of claim 12, wherein the target structure consists of only thethree parts.
 18. The target structure of claim 17, wherein the differentbias values in the first and second directions of the three partscomprise (+d, +d), (−d, +d), and (+d, −d), where d represents a relativeoffset distance between each of the respective three parts.
 19. Thetarget structure of claim 17, wherein the different bias values in thefirst and second directions of the three parts comprise (+d, +d), (−d,+d), and (0, −d), where d represents a relative offset distance betweeneach of the respective three parts.